1. Technical Field
The present invention relates to valves, and in particular, to valves for draining the combustion area of turbine engines.
2. Description of the Related Art
Gas turbine engines are commonly used in power generation and propulsion applications. Gas turbine engines have a set of rotating turbine blades to compress air leading to one or more combustors into which fuel is injected and ignited. Fuel is delivered through metering orifices to burners in the combustors under pressure through a fuel line. Combustion of the fuel turns a downstream set of blades, used for energy extraction or propulsion, and which can be used to drive the compressor blades.
Gas turbine engines typically burn gaseous fuel, such as natural gas, and ignite using liquid fuel (such as diesel fuel). Some turbine engines are capable of sustained operation burning either gas or liquid fuel. Cost, clean burning and other considerations usually dictate that natural gas is the primary fuel for sustained operation, particularly for power generation applications. However, as mentioned, liquid fuel is often used for ignition or as a backup fuel supply in the event of a disruption in the natural gas line.
Gas turbine engines are designed for lengthy continuous operation, particularly in the case of power generation. Since they ordinarily run on natural gas, the liquid fuel system is often left unused for long periods. The heat and pressure associated with combustion of the gaseous fuel can cause “coking”, which occurs when the volatile components of the fuel are driven off by heat such that only a tarry deposit remains. Among other things, the coke deposits can build up on the liquid fuel burners and/or inhibit proper conduit of the liquid fuel when the engine is returned to fuel mode. When this happens at ignition, the combustion can fail causing a false start of the turbine. This false start can leave unspent liquid fuel in the combustor(s). Large gas turbines, such as those used in power generation, can have several combustion cans which can consume many gallons (35 gallons or more in some cases) of liquid fuel at ignition. This large volume of unspent liquid fuel must be drained from the combustors before ignition can be attempted again in order to prevent excessive combustion. Drain lines and collection wells are provided for this purpose.
Valving is used to open the combustors to the drain after a false start and keep the drain isolated during operation so that compression can be sustained. Conventional valves have several disadvantages particularly with regard to high pressure drops and resisting the effects of contamination. For example, common gate or globe type valves used for this purpose require the drained fuel to follow a non-linear path as it passes through the valve. This change in direction of the flow presents at least two distinct problems, namely, it causes a higher pressure drop across the valve and presents locations for the liquid fuel to collect, particularly given that the drain flow is usually not under pressure. Since the collected fuel is still in a high heat environment, it can cause a safety concern if combined with air, and it also can lead to coke deposits on the valve.
Ball-type valves are sometimes employed to allow for straight through flow of the drained fuel. The straight flow reduces the pressure drop and also alleviates some of the pooling of fuel inside the valve. However, it is still susceptible to other adverse effects of contamination. In particular, contaminants and coking deposits can arise on the non-sealing surface of the ball valve during operation (when the valve closes the drain). However, the deposit can be transferred onto the sealing surface of the valve seat when the valve is actuated. Specifically, as the ball is rotated to open, the build-up on the outer wall of the valve can rub against the valve seat. Once this occurs, the seal is compromised and turbine compression can be diminished by leakage through the valve to the drain. This in turn reduces the efficiency of the engine.
Accordingly, an improved drain valve is needed for draining liquid fuel from the combustors of gas turbine engines.